1. Field of the Invention
The present invention relates to the process of controlling the curing of composite materials and, more particularly but not by way of limitation, to the computer controlled selection of cure cycles during the curing process of composite materials in a curing vessel such as an autoclave.
2. Discussion of the Related Art
In recent years there has been dramatic growth in the utilization of polymers for a wide variety of applications. As with any material, one must consider carefully the issues of processing and quality assurance for the fabricated component. While a great deal of experience has been developed for metallic parts, there is much less information available for composite laminates (wherein the term "composite" refers to any thermosetting material constructed of at least two constituent materials). Of particular concern is the complexity of the fabrication process and susceptibility of these structures to both the same types of bulk defects that are seen in conventional materials (such as voids, porosity, and inclusions) as well as an array of more subtle defects (such as overcure/undercure, moisture degradation, weakly bonded interfaces and delamination) unique to composites. Accordingly, new techniques must be available to insure reliability. As performance is dependent upon processing conditions, considerable benefits will result from a method integrating nondestructive evaluation (NDE) into the manufacturing phase.
One example of a composite is graphite/epoxy laminate. Graphite/epoxy is the most commonly used material system and the preparation of vacuum bag layup and application of autoclave cure cycle follows fairly well-established procedures. Nonetheless, the design of a cure cycle is still generally performed in an ad hoc fashion after numerous trials Currently, neither rigorous cure models nor real time nondestructive tests are utilized to any great extent in the composites industry. The nondestructive testing of a part after the fabrication is used today as the principal means of quality assurance. Hence, the ability to analyze the fabrication of graphite/epoxy systems with real time interactive NDE in conjunction with fundamental cure models would be a significant contribution to the existing science of composite manufacturing.
Nondestructive (particularly ultrasonic) test methods offer a powerful means for characterizing material microstructure. Historically, the use of these techniques has been basically confined to gross defect identification based on the large impedance (hence acoustic reflections) differences between flawed and unflawed materials. For metals, this approach has been acceptable because structurally critical flaws (cracks, voids, inclusion) are readily detected with this approach. Composites, while subject to many of the same gross flaws as metals, are susceptible to other significant flaws as well. Because composites are an engineered microstructure, defects in composites are often significantly more subtle than the gross defects described above and are virtually undetectable with conventional approaches Typically, these defects are in the number, type, distribution and efficiency of the microconstituents of the composite. Unfortunately, the bulk of composite NDE today is performed using the amplitude based techniques adapted from metals testing and which are totally inappropriate for finding these critical defects.
Most of the quality control/NDE test procedures currently being used evolve from previous experience in the metals and plastic industries. Most companies divide their activities into quality assurance and nondestructive testing with the bulk of the quality assurance activity devoted to qualifying incoming material to insure that it meets established standards and NDT (Non-Destructive Testing) for defect identification. However, the bulk of the tests being performed suffer from one or more of several important drawbacks:
They are destructive.
Only a limited number of samples are tested.
They are insensitive to certain critical changes in microstructure.
They are performed after fabrication where little or nothing can be done to salvage a flawed component.
Considerable benefits will accrue from a thorough integration of advanced nondestructive measurement science techniques into the production process. Potential benefits are available in the following areas:
Reduced Risk. In a typical aerospace operation, it is not unusual for a batch of composite material to cost in excess of fifty thousand dollars. A critical failure in the process may cause the entire batch to be scrapped.
Reduced Process Time and Processing Steps. One of the optimization factors driving the control process is to minimize the time required to process the part. The process time can be reduced by up to one half the original time in some cases. Because of the uncertainty in the cure status of materials, several steps are often added to the process to assure, for example, that gases have been removed prior to heating the resin above its cure initiation temperature. Some of these steps could be combined if the cure status were known.
Improved Process Capability. The ability to process materials here-to-for considered too difficult to manage, opens a number of options to the designer. If it is possible to build a part which previously could not be built due to the technological limitations, there is significant economic potential both for a new product and for the development of a competitive edge.
Improvement in Quality. The ability to sense and predict the cure status of the resin allows the manufacturer to constantly adjust the process for optimum part quality. With previous technologies, the composite producer has had only limited mean to determine and moderate the cure or consolidation status of the part.
While composites continue to grow in their importance in virtually every facet of the aerospace industry, the bulk of the research activity has been devoted to the development of improved fiber and matrix systems (materials science) and understanding the mechanics of anisotropic, laminated structures (mechanics). However, the research in the processing area, where both materials and mechanics expertise are required, has been limited. While there has been some success in process modeling, little use has been made of this information to date in developing interactive process control systems for composite manufacture. Most current approaches rely upon empirical relations among the pertinent variables (temperature, pressure, degree of cure, viscosity) rather than a unified model of material behavior based on fluid/solid mechanics and heat transfer principles. However, these techniques are limited to the comparison of real time cure characteristics to a preset cure cycle profile which does not change during the curing process (see U.S. Pat. Nos. 4,455,268, 4,515,545, and 4,559,810 issued to Hinrichs and Thuen).
A significant improvement in existing process control methodology would result from incorporating suitable analytic models. The predictions of future behavior of the cure process could then be used to make decisions about the future course of the cure process. Therefore, the object of the present invention is to provide a methodology employing analytic models for continuously selecting new optimal cure cycles during the curing process in response to actual material behaviors which occur over time within the composite part during the curing process.